1. Field of the Invention
The present invention relates to battery powered selective call systems such as a paging systems and particularly to such a system which provides enhanced battery power conservation.
For convenience of description the present invention will be described in the context of the CCIR Radiopaging Code No. 1 (otherwise known as POCSAG) which is described in detail in "The book of the CCIR Radiopaging Code No. 1" published by the Radiopaging Code Standards Group (RCSG) and obtainable from British Telecom, Radiopaging, London, England. However the invention may be applied to systems using other digital paging codes.
2. Description of the Related Art
As is well known POCSAG has an inherent battery power conservation signal format which comprises for each new transmission a preamble of 576 bits, which serves to enable a paging receiver to obtain bit synchronization and a plurality of concatenated batches of bits. Each batch comprises a synchronization codeword, which enables a pager to obtain word synchronization, and 8 frames each comprising 2 codewords. A control device in a pager is preprogrammed with the number of the frame in which an alert signal comprising the pager's identity codeword (RIC) will, when necessary, be transmitted. Accordingly, after achieving bit synchronization, the receiver section of a pager only requires to be energised for two periods in each batch; firstly, to receive the synchronization codeword and, secondly, for the duration of the pre-designated frame in order to be able to receive its RIC, if transmitted. During the time intervals between the end of the synchronization codeword and the start of the pre-designated frame, assuming that it is not the first frame, and between the end of the pre-designated frame and the end of the batch, the receiving section can be de-energized for a period equivalent to 7 frames thereby achieving a battery saving duty cycle of 1:8. A clock included in the control device controls the energization of the receiving section. For pocket size pagers which are able to hold relatively large capacity batteries, for example two size AAA batteries, the battery power conservation feature of the POCSAG signal format enables an acceptable battery life to be achieved.
However for some special applications, for example where a pager is built into a wristwatch, relatively small and expensive lithium batteries are used to energize the pager. Accordingly there is a desire to increase the battery economizing aspect of the pager if necessary at the expense of service.
Various proposals are known to enhance the conservation of battery power. For example PCT specification WO 90/06634 discloses conserving power during the transmission of predetermined signals such as the synchronization codeword. This method requires examining the first 8 bits of the 32 bit synchronization codeword as received, and if it contains less than two bit errors, the complete codeword is assumed to have been received correctly and the control device switches off the receiving section until the pre-designated frame in the same batch. Such a method requires resetting the clock interval. However, if the first 8 bits contain two or more bit errors, then the receiving section has to remain energised to receive the entire synchronization codeword and either the remaining 24 bits are checked for errors or the entire codeword is checked for errors.
Another proposal, disclosed in PCT specification WO 88/05948, is the creation of what may be termed a "superbatch" comprising N POCSAG batches, where N=16. In order to be able to identify each batch in a superbatch, the synchronization codeword is given an identity comprising say a four bit binary word ranging from 0 to 15. The synchronization identity is appended to the synchronization codeword.
The control device in the pager is then preprogrammed to energize the receiving section to receive one or more of the N synchronization codewords in a superbatch together with the pre-designated frame in the associated batch or batches. Thus in the case of say a wristwatch pager, the receiving section may be energized firstly to receive the synchronization codeword and secondly for the duration of a subsequent frame in order to be able to receive its RIC, if transmitted, in one batch in every 16 batches, thus achieving a battery saving duty cycle of 1:128. In other applications, more than one batch in a superbatch may be used, which however leads to a corresponding reduction in the battery saving duty cycle. In the case of a normal pager which is able to operate in all the batches, it has to receive and decode all the synchronization codewords and their identities. That means that not only must the receiving section be powered up longer in order to receive the additional bits, but also the option disclosed in WO 90/06634 cannot be used. Furthermore, a paging system in which synchronization codeword identities are appended to the synchronization codeword is not compatible with the normal POCSAG system, which means that this proposal is not transparent to users not wishing to apply the enhanced battery power conservation technique.